Developing colloid resists with substantive dyes



Patented Nov. 12, 1946 OFFICE DEVELOPING COLLOID RESISTS WITH SUBSTANTIV E DYES Frank T. Powers, Glen Cove, NY.

No Drawing. Application February 26, 1944, Serial No. 524,074

8 Claims.

The present invention relates to anew and improved photomechanical process, and more particularly to an improved process of preparing photoengraved printing surfaces.

Objects and advantages of the invention will be set forth in part hereinafter and in part will be obvious herefrom, or may be learned by practice with the invention, the same being realized and attained by means of the processes, steps and combinations pointed out in the appended claims.

The invention consists in the novel steps, processes, combinations and improvements herein shown and described.

The present invention has for its object the provision of a novel and improved process of preparing a photomechanical printing surface, such as a halftone or line engraving on zinc, copper or other metal. A further object is the provision of an improved process of photoengraving by which a more durable resist is prepared, and in which the exposure time for the photosensitive resist is rendered less critical. Still another object is the provision of a process of preparing photoengravings in which the edges of the varirendered unusually resistant to the mordant or etching material. The invention also provides a process of forming a photomechanical image in which the image is developed more cleanly than 2 ous areas of the developed resist are hardened or usual and without swabbing or similar treatment such as has usually been necessary. The invention further provides a process in which a durable resist may be produced ready for etching without burning-in of the exposed and devel- P oped image on the metal plate.

In accordance with the process of the present invention, the surface to be transformed into a printing surface or other photomechanical member is coated with a thin layer, of usual thickness,

of a light sensitive composition containing film forming, mordant-resisting material which hardens on exposure-to light, while the unexposed material remains solubl and may be developed away prior to etching. This photosensitive material may comprise a normally soluble, lighthardenable mixture of a gelatinous colloid, such as gelatine or glue, and a photosensitive hardening agent, such as chromic acid salt o one of the less usual photosensitive, gelatine-hardening salts, such as the vanadates, tungstates, molybalthough the exposure given under such a negative may be much greater than is usual and is preferably from two to four times a normal exposure. Also, preferably, the photosensitive mixtur includes salts which further assist in the hardening of the mixture under the influence of the light exposure, and where gelatine is used with chromic acid salts, additions of ferric ammonium citrate are highly desirable.

After the sensitized surface has been thus exposed, it is developed by immersion in a solvent for the unexposed colloid, and where soft gelatine is used, the development may be carried out in running warm or hot water or in a hot aqueous bath having a slight detergent action on the unexposed areas of the light sensitive layer.

During or after the development of the image, the image is preferably dyed so that it may be more readily observed and judged, and is treated, preferably by dyeing, so that the image remaining after development and particularly the outer edges of the image may be additionally hardened and thus rendered more resistant to the action of the mordant which tends to cause the undeveloped areas of the image to be removed from the surface of the member to be etched.

The development of the image, its dyeing, clearing and preparation for hardening may be simultaneously accomplished by immersion of the exposed member in a heated bath comprising an aqueous solution of a substantive dye. The substantive dyes ar preferred for this purpose inasmuch as they exert a detergent action on the surface and remove from it the scum which might otherwise interfere with the production of a clean image, they may be subsequently treated with chromic acid toharden the gelatine, and due to the relatively larg size of the dye molecules with reference to the dimensions of the gelatine lattice, the dye solution is dialyzed by the hardened areas of the developed resist and is thus more strongly adsorbed by the edges of the hardened areas or half-tone dots. The substantive dyes, of course, are fast dyes for gelatine and are thus not subject to bleeding during the subsequent stages of the treatment.

The surface is then rinsed in water and is immersed in a hardening bath which may comprise a dilute solution of chromic acid, or other hardening material rendered active on the gelatine by the presence of the adsorbed dye or other agent. Other compounds which may be used in place of the more desirable chromic acid are the acids of vanadium, tungsten, molybdenum and manganese, chrome alum, tannic acid and sodium bisulfite.

After this treatment, the surface may be heated to burn-in the resist, if desired, although this is not essential even for the etching of Zinc with nitric acid. Ihereafter the surface is etched, powdered and reetched in the usual manner with a suitable mordant such as nitric acid for zinc or ferric chloride solution for copper, and after rinsing and drying the surface is ready for printing in the usual manner.

As used herein, the term substantive dye is used in its usual meaning as defining that group of dyes which dye cotton without the application of a mordant or fixing agent. A

The process of the present invention which has been set forth above is highly advantageous from many different points of view; the exposure to the halftone or line negative need not be timed precisely, gelatine is used in place of the more expensive and less uniform glue, the developed image is cleaner and is less subject to scumming or veiling, the exposed and developed image is harder, and more resistant to the mordant and is rendered even more resistant to the mordant by the subsequent treatment with a hardening agent activated by the adsorbed dye, the image is dyed with a fast dye, the outer edges of the dots or lines forming the image are hardened more than the remainder of the dots and there is therefore less likelihood of lifting of the image or undercutting during etching, the image is sufficiently resistant so that zinc may be etched with nitric acid without burning-in, the photosensitive resist may be used for the production of printing plates for letterpress or intaglio printing in one or more colors, and may be applied either to flat plates or printing cylinders, and, in many of its aspects, the process is applicable to the production of planographic and rotary photogravure printing members as well as other surfaces produced by photomechanical etching.

By the process of the present invention, and as exemplified in the production of process color plates for multi-color printing the halftone dots in the printing member as finished ready for printing are a substantially exact reproduction of the halftone dots in the negative, as contrasted with the great reduction in area of the dots of the printing surface usually experienced, and which has been measured my Amstutz to be as great as 50% as the result of twenty minutes etching.

It will be understood that the foregoing general description and the following detailed description as well are exemplary and explanatory of the invention but are not restrictive thereof.

Referring now in detail to the process of the present invention particularly as applied to the production of halftone or line photoengravings on copper or zinc plates for letterpress printing:

The photosensitive resist preferably comprises:

Soft gelatine About 400 grams Potassium bichromate About 80 grams Ferric ammonium citrate About 80 grams Dissolved in water About 4 litres With respect to certain portions of the process, other colloids than gelatine may be used which are rendered insoluble on exposure to light, but a soft gelatine is preferred and is preferably sufficiently soft so that it may be flowed onto the printing surface at room, or only moderately warm, temperatures.

Other chromic acid salts may be substituted for the potassium bichromate in the usual manner,

and less desirably other light sensitive, gelatine hardening compounds may be employed, such as ammonium molyb-date, sodium tungstate or other 5 suitable salts of the acids of vanadium, manganese, molybdenum or tungsten,

Ferric ammonium citrate is a desirable, but optional, addition and may be omitted if not required' by the etching procedure to be employed.

Such a mixture is flowed on the flat cop-per, zinc or other flat printing plate or surface and distributed evenly thereon, as by whirling in the case of a fiat plate, after which it is dried in the usual manner.

The sensitized surface is then placed in contact with a halftone or line negative and is exposed in the usual manner, although the exposure is preferably from two to four or more times the exposure with conventional chromate-glue resists. l

The fully exposed surface is then preferably immersed in a heated solution. of a substantive dye which, exemplarily may be a 1% aqueous solution of the substantive dye, brillantbenzoechtviolett (Schultz 1931, edition ,No. 610, BL, 2 RL), heated to about 120 to 140 F. and preferably 125 F. and is allowed to remain in the developer until the image is fully developed, which generally requires only a few moments.

As an alternative to the development in the dye solution, the image may be dyed by immersion in a dye solution, which is preferably a fast dye, may be developed by running water at 120 to 140 F., subjected, if desired to a detergent action such as a solution of a preferably non-alkaline sulfonated detergent compound, and then, if desired, treated with a compound to be adsorbed by the remaining areas of the developed image and which will be dialyzed by the gelatine 0 and will subsequently activate the chromic acid hardening bath. However, the single step of developing, dyeing and impregnating the gelatine resist is preferred.

The printing member is then preferably rinsed to remove the excess dye, and is then immersed in an aqueous solution of a chromic acid compound, such as 3% chromic acid, for which, less desirably other hardening compounds might be substituted, such as tannic acid. sodium bisulfite,

chrome alum, and the acids of chromium-like metals consisting of chromium, vanadium, tungsten, manganese and molybdenum.

Immersion of the developed and treated image in the chromic acid bath for a short period of 65 time causes the image to be hardened and as the adsorbed material is more concentrated at the edges of the dots or lines of the developed image, these edges are hardened to a greater extent than the interior of the dots or lines, thereby rendering the dots and lines more resistant to the action of the subsequent etch.

The plate or printing member is then removed from the hardening bath and is rinsed in water, and if desired, may be heated to burn-in the image or enamel, although this step is generally unnecessary where the foregoing detailed steps have been fully followed.

After the first etch has been completed, either with nitric acid on a zinc plate, or ferric chloride solution on a copper plate, and particularly with line engravings on zinc letterpress plates, the surface of the plate may be powdered in the usual manner with dragons blood or other resin which is then melted, and the further etchin and treat- 76 ment of the plate continued in the usual manner.

Due to the exceptional durability of the resist produced by the process of the present invention, the etching of the plate may be carried far beyond normal depth, and in many instances the etching may be carried to such an extreme depth that routing of even relatively large areas is rendered unnecessary.

The invention in its broader aspects is not limited to the specific processes and steps shown and described but departures may be made therefrom within the scope of the accompanying claims without departing from the principles of the invention and without sacrificing its chief advantages.

What I claim is:

1. The process of photoengraving which includes coating a surface to be etched with a light sensitive resist including a gelatinous colloid sensitized by a chromic acid salt, exposing the resist to a light image, developing the exposed resist and treating the exposed portions of the resist with a substantive dye and a hardening compound reactive With the dye.

2. The process of photoengraving which includes coating a surface to be etched with a light sensitive resist including a gelatinous colloid sensitized by a chromic acid salt, exposin the resist to a light image, developing the exposed resist with a substantive dye and then a chromic acid compound.

3. The process of photcengraving which includes coating a surface to be etched with a light sensitive resist including a gelatinous colloid sensitized by a chromic acid salt, exposing the resist to a light image, developing the exposed resist in a substantive dye solution and treating the remaining portions of the resist with a hardening agent.

cludes coating a surface to be etched with a soft gelatine layer rendered light sensitive by a hardening compound, exposing the resist to a light image, developing the resist in Water from F. to F., dyeing the remaining portions of the resist with a substantive dye and treating said portions with a chromium containing hardening agent.

5. The process of photoengraving which includes coating a surface to be etched with a soft gelatine layer rendered light sensitive by a subchromic acid salt, exposing the resist to a light image, developing the resist in water from 120 F. to 140 F., dyeing the remaining portions of the resist with a substantive dye and treating said portions with a chromium containing hardening agent.

6. The process of photoengraving which includes coating a surface to be etched with a soft gelatine layer rendered light sensitive by a hardening compound, exposing the resist to a light image, developing the resist in water solution of a substantive dye at 120 F. to 140 F., and treating the remaining portions of the resist with a chromium hardening compound.

7. The process of photoengraving which includes developing an exposed photosensitive resist including a gelatinous colloid sensitized by a chromic acid salt in a solution of substantive dye having a detergent action on the resist.

8. The process of photoengraving which includes developing an exposed photosensitive resist including a gelatinous colloid sensitized by a chromic acid salt in a solvent for the unexposed resist, and then treating the remaining portions with a substantive dye solution.

FRANK T. POWERS. 

